Apparatus and method for transmitting/receiving data in multi-user multi-antenna communication system

ABSTRACT

An apparatus and method for transmitting/receiving data in a multi-user multi-antenna communication system is provided. A data transmitting apparatus and method computes a TX filter of a transmitter in an improved scheme and transmits the computed TX filter to a receiver in a one-way channel sounding scheme. A data receiving apparatus and method receives the TX filter over a channel and uses the product of the received TX filter and a channel matrix as an RX filter.

PRIORITY

This application claims priority under 35 U.S.C. §119 to a Koreanapplication filed in the Korean Intellectual Property Office on Oct. 17,2005 and allocated Serial No. 2005-97718, the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an apparatus and method fortransmitting/receiving data in a multi-user multi-antenna communicationsystem. In particular, the present invention relates to a datatransmitting apparatus and method for computing a transmit (TX) filterof a transmitter in an improved scheme and transmitting the computed TXfilter to a receiver in a one-way channel sounding scheme; and a datareceiving apparatus and method for receiving the TX filter over achannel and using the product of the received TX filter and a channelmatrix as a receive (RX) filter.

2. Description of the Related Art

Research has been conducted on Multiple Input Multiple Output (MIMO)channels over ten years. In addition, researches are being conducted ona multi-user multi-antenna communication system. The use ofmulti-antenna in a transmitter increases spectral efficiency and alsothe use of a multi-antenna in a receiver further increases spectralefficiency. Research on the multi-user multi-antenna communicationsystem is intended to apply the above fact to a multi-user communicationsystem.

A MIMO communication system establishes a multi-link between atransmitter and a single user, thereby increasing spectral efficiency.In the MIMO communication system, only one user can access a givenresource among channel resources (e.g., subcarriers, spreading codes,and cell sectors) at a time. That is, a MIMO link (i.e., independentdata streams) exists between a transmitter and only one receiver at agiven time. On the other hand, the multi-user multi-antennacommunication system allows a plurality of users (terminals) to accessthe same resource simultaneously, and independent data streams occurbetween a transmitter and a plurality of user receivers. Themultiplexing scheme used for this is called “multi-user SpatialMultiplexing (SM)”.

FIG. 1 is a schematic block diagram of a multi-user multi-antennacommunication system where communications are performed between atransmitter having a plurality of TX antennas and a plurality ofreceivers each having a plurality of RX antennas.

Referring to FIG. 1, a base station (BS) transmitter 110 having N numberof TX antennas (simply referred to as “N TX antennas”) communicates withK user receivers 120, 130 and 140 each having a plurality of RXantennas. The transmitter 110 transmits independent data streams to thereceivers 120, 130 and 140 by using a multi-user SM scheme.

A MIMO broadcast channel, an example of a multi-user multi-antennacommunication channel, is a downlink (DL) channel of a cellular networkwhere a base station (transmitter) uses multiple TX antenna.

An example of a theoretical scheme for a MIMO broadcast channel in acellular system is disclosed in M. Costa, “Writing On Dirty Paper”, IEEETransactions on Information Theory, Vol. 29, No. 3; pp. 439-441, May1983. This theoretical scheme is, however, unrealistic because it isbased on the assumption that a transmitter and every user receiveraccurately know the channels of all user receivers. In addition, theabove theoretical scheme is highly complex because it uses non-linearprecoding techniques.

FIG. 2 is a block diagram of a multi-user multi-antenna communicationsystem where a transmitter uses a plurality of TX antennas and a TXfilter and a plurality of receivers each using a plurality of RXantennas and an RX filter. For actual communication, a transmitter usesa TX filter and each receiver uses an RX filter.

Referring to FIG. 2, the transmitter includes SM TX filters 210, 220 and230 that incorporate TX filters M_(l), M_(k), . . . , M_(n),respectively. Likewise, the receivers respectively include SM RX filters240, 250 and 260 that incorporate RX filters W_(l), W_(k), . . . ,W_(n), respectively.

A multi-user SM scheme, disclosed in Lai-U Choi and Ross D. Murch, “ATransmit Preprocessing Technique for Multiuser MIMO Systems Using aDecomposition Approach”, IEEE Transactions on Wireless Communications,Vol. 2, No. 4, pp. 773-786, July 2003, is based on the assumption thatsome entity (terminal) knows channel matrixes of all users (terminals)and can compute TX/RX filters for optimization of communicationperformance.

The disclosed multi-user SM scheme is, however, silent on which entitycan compute the TX/RX filters, and on how the computed TX/RX filterinformation (knowledge) can be transmitted from a transmitter toreceivers. If information (knowledge) about all the respective channelmatrixes between the transmitter and the receivers is available to thereceiver, it is called “global channel information (knowledge)”. Thisis, however, also impossible in an actual system.

FIG. 3 is a graph illustrating spectral efficiencies depending on thetype of algorithm used in a multi-user multi-antenna communicationsystem. In FIG. 3, local Channel State Information (CSI) refers to arealistic case where each receiver knows only its own channel matrix.Global CSI refers to an unrealistic case where each receiver knowschannel matrixes of all receivers. Partial CSI refers to a case where atransmitter uses some measure of channel. quality, which indicates CSIsuch as signal-to-noise ratio (SNR). Complete CSI refers to a case wherea transmitter uses the complex entry of CSI in itself.

As can be seen from FIG. 3, the frequency efficiency in the case oftransmission to a single user (single-user closed loop, local CSI,complete CSI), as illustrated mark +, is lower by 4.6 bits/sec/Hz thanthat in the case of sum-capacity (nonlinear precoding scheme; globalCSI, complete CSI), as illustrated mark ∇, which is the theoreticalmaximum data rate of a multi-user communication having four TX antennas,four users, and four RX antennas. Although not illustrated in FIG. 3,the frequency efficiency decreases when multiplex transmission isperformed on user receivers but a transmitter and a receiver use partialCSI and local CSI, respectively.

The spatial efficiency in the case of a coordinated beamformingalgorithm, as illustrated mark o, is lower by 0.7 bits/sec/Hz than thatin the case of the sum-capacity. The use of the coordinated beamformingalgorithm makes it possible to design an effective method oftransmitting information about RX filters to user receivers bytransmitting only one layer to each user receiver, as disclosed in B.Farhang-Boroujeny, Q. Spencer and L. Swindlehurst, “Layering Techniquesfor Space-Time Communication in Multi-User Networks,” in Proceedings ofIEEE Vehicular Technology Conference (VTC'03 Fall), Orlando, Fla., Oct.,6-9, 2003, Vol. 2, pp. 1339-1343.

The coordinated beamforming algorithm is now described in detail.

First, an algorithm for computing TX/RX filters follows:

Computation Phase

A_(i) represents the i^(th) column of any matrix A. When the matrix A isSingular Value Decomposition (SVD)-processed, A=UDV*. Here, U and V areunitary matrixes and D is a singular value of the matrix A with diagonalelements arranged in descending order. The principal left singularvector of the matrix A is denoted as U_(l) that is the first column ofthe matrix U.

The following computation is performed on the assumption that a basestation has a complete CSI.

Initialization for k = 1:K H_(k) = UDV   SVD W_(k) = U₁ end Repeattimes: Effective channel vector computation for k = 1:KH_(eff, k) = W_(k)^(*)H_(k) end Update of TX/RX filters for k = 1:KH_(stacked, k) = [H_(eff, 1)^(T) . . . H_(eff, k − 1)^(T)  H_(eff, k + 1)^(T) . . . H_(eff, K)^(T)]^(T)H_(stacked,k) = U_(k)D_(k)V_(k)   SVD M_(k) = V_(k,N)$W_{k} = \frac{H_{k}V_{k,N}}{{H_{k}V_{k,N}}}$ end End of iterations

where SVD represents singular value decomposition, D is a diagonalmatrix, and U and V are unitary matrixes. H_(eff,k) represents aneffective channel matrix of a user receiver k that the receiver actuallyexperiences, and H_(stacked,k) represents effective channels ofreceivers of all users other than the user receiver k. M_(k) is a TXfilter matrix for the user receiver k, W_(k) is an RX filter matrix forthe user receiver k, and V_(k,n) is a singular vector in the num spaceof H_(stacked,k). T represents a transpose, and * represents a complexconjugate transpose.

As the algorithm is iterated to convergence, a transmitter performszero-forcing beamforming (ZFBF) on each user receiver based on aneffective channel matrix containing an RX filter.

Although not indicated in the above algorithm, SVD is again used tocalculate W_(k) from M_(k). As a result, the above algorithm performsSVD on each user receiver twice during the computation of the TX/RXfilters and performs SVD once during the initialization, which causes acomplexity problem. Moreover, the base station knows the optimal TX/RXfilters for each of N user receivers but the user receivers do not.However, the above algorithm is silent on a technique for informing thereceivers of the optimal RX filters. What is therefore required are: (a)a scheme for informing the receivers of the optimal RX filters;

(b) a data transmitting apparatus and method for a multi-antennacommunication system that can provide a simpler scheme for computing aTX filter and an improved scheme for enabling a receiver to easilycompute an RX filter; and (c) a data receiving apparatus and method fora multi-antenna communication system that can efficiently receive a TXfilter computed by a transmitter. Finally, what is required is a datatransmitting/receiving apparatus and method for a multi-usermulti-antenna communication system that can reduce the systemcomplexity, increase the spectral efficiency and be implemented evenwhen a receiver does not know the channels of other receivers.

SUMMARY OF THE INVENTION

An object of the present invention is to substantially solve at leastthe above problems and/or disadvantages and to provide at least theadvantages below. Accordingly, an object of the present invention is toprovide a data transmitting apparatus and method for a multi-antennacommunication system that can efficiently compute a TX filter.

Another object of the present invention is to provide a datatransmitting apparatus and method for a multi-antenna communicationsystem that can efficiently transmit the computed TX filter to areceiver.

A further object of the present invention is to provide a data receivingapparatus and method for a multi-antenna communication system thatreceives the computed TX filter from the transmitter and computes an RXfilter using the received TX filter.

According to one aspect of the present invention, there is provided atransmitter for a multi-user multi-antenna communication system,including a Spatial Multiplexing (SM) TX filter unit for computing TXfilters for a plurality of user receivers; and a plurality of antennasfor transmitting the computed TX filters to the corresponding userreceivers.

According to another aspect of the present invention, there is provideda receiver for a multi-user multi-antenna communication system,including one or more antennas for receiving a TX filter from atransmitter; and an SM RX filter unit for normalizing the product of thereceived TX filter and a channel matrix to obtain an RX filter.

According to a further aspect of the present invention, there isprovided a method for transmitting data from a transmitter in amulti-user multi-antenna communication system, the method includingcomputing TX filters for a plurality of user receivers; and transmittingthe computed TX filters through a plurality of antennas to thecorresponding user receivers in a one-way channel sounding scheme.

According to still another aspect of the present invention, there isprovided a method for receiving data at a receiver in a multi-usermulti-antenna communication system, the method including receiving a TXfilter from a transmitter through one or more antennas; normalizing theproduct of the received TX filter and a channel matrix to obtain an RXfilter; and decoding the received data using the obtained RX filter.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic block diagram of a typical multi-usermulti-antenna communication system where communications are performedbetween a transmitter having a plurality of TX antennas and a pluralityof receivers each having a plurality of RX antennas;

FIG. 2 is a block diagram of a typical multi-user multi-antennacommunication system where a transmitter uses a plurality of TX antennasand a TX filter and a plurality of receivers each use a plurality of RXantennas and an RX filter;

FIG. 3 is a graph illustrating spectral efficiencies depending on thetype of algorithm used in a multi-user multi-antenna communicationsystem;

FIGS. 4A and 4B are flow diagrams illustrating a communication procedurebetween a transmitter and a receiver in a multi-user multi-antennacommunication system according to the present invention;

FIGS. 5A and 5B are flowcharts illustrating a transmission procedure ina multi-user multi-antenna communication system using an algorithmaccording to the present invention;

FIG. 6 is a graph illustrating spectral efficiencies depending on thenumber of TX antennas in a multi-user multi-antenna communication systemusing an algorithm according to the present invention; and

FIG. 7 is a graph illustrating spectral efficiencies depending on dataSNRs in a multi-user multi-antenna communication system using analgorithm according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described hereinbelow with reference to the accompanying drawings. In the followingdescription, well-known functions or constructions are not described indetail since they would obscure the invention in unnecessary detail.

The present invention provides a data transmitting apparatus and methodfor a multi-user multi-antenna communication system that computes a TXfilter efficiently and transmits the computed TX filter to a receiver ina one-way channel sounding scheme; and a data receiving apparatus andmethod for a multi-user multi-antenna communication system that receivesthe TX filter over a channel and uses the product of the received TXfilter and a channel matrix as an RX filter.

The present invention is premised on a communication system where atransmitter knows DL CSI on a receiver. In the following description, aTime Division Duplexing (TDD) multi-user multi-antenna communicationsystem where a BS transmitter can obtain Down Link (DL) Channel StateInformation (CSI) by estimation of an uplink (UL) channel is taken as anexample.

The present invention considers a multi-user multi-antenna communicationsystem where a BS transmitter has N TX antennas and a user receiver kamong a plurality of user receivers has N_(k) RX antennas, asillustrated in FIG. 1. A channel between the transmitter and the userreceiver k is represented by a matrix H_(k). When a channel changesslower than DL/UL frames, it can be said that the channel is constantfor several frames and is the same throughout a downlink and an uplink.In this case, a BS and user receivers estimates the same channel. The BScan estimate all channel matrixes H_(k) (k=1, . . . , k,) between the BStransmitter and the user receivers using a data transmission or uplinksounding pilots transmitted by the user receivers. In other embodimentthe user receiver k can estimate a matrix H_(k) and the user receiver kreports the estimated H_(k) to the BS. The transmitter and the receiversperform the channel estimation using a well-known scheme.

Signals are transmitted from the BS to N user receivers, which is thesame as the number of the TX antennas as illustrated in FIG. 2. Ascheduling algorithm is used to select a user receiver. The schedulingalgorithm allocates frequency, time, and space in consideration offactors such as the amount of work, delay, and Quality of Service (QoS),and uses a scheme for maximizing transmission efficiency. Thisscheduling algorithm may be conventional, and a scheduler forimplementing the scheduling algorithm is installed preceding the SM TXfilters 210, 220, . . . , 230 in FIG. 2.

After performing the scheduling algorithm, the transmitter performs thefollowing computation phase.

Computation Phase

A simplified algorithm for computing a TX filter M_(k) is used to reducethe complexity in the case where one layer is transmitted to each userreceiver. The present invention simultaneously obtains N TX filters forN user receivers by the pseudo-inversion of effective channel matrixes.By doing this, the complex SVD operation, which needs to be performed ona digital signal process twice in each iteration phase in each userreceiver, can be omitted to simplify computation.

In addition, the SVD operation necessary for initialization in each userreceiver can be omitted by setting an initial RX filter to a randomunitary vector.

The simplified algorithm which, reduces the complexity involved, iscomprised of the following steps:

Firstly, the RX filter is initialized to a random value for each of theK users.

Secondly, the following computation is repeated Ni times: Compute theeffective channel matrix using the current RX filters. Then compute thepseudo-inverse of the effective channel matrix using the current RXfilters and the channel matrices. Then for each user k, update thecurrent TX filter as the normalized k-th column of the invertedeffective channel matrix. Update the RX filter for each user k as theproduct of the channel matrix of user k by its current TX filter,followed by normalization.

The simplified algorithm which, reduces the complexity involved follows:

Initialization for k = 1:K W_(k) = random unitary vector of size N_(k) ×l end Repeat N_(i) times:Define  .H_(eff) = [(W_(l)^(*)H_(l))^(T) . . . (W_(k)^(*)H_(k))^(T) . . . (W_(K)^(*)H_(K))^(T)]^(T)for k = 1:K$M_{k} = \frac{H_{{eff},k}^{\dagger}}{H_{{eff},k}^{\dagger}}$Compute  .H_(eff)^(†) = H_(eff)^(†)(H_(eff)H_(eff)^(†))⁻¹$W_{k} = \frac{H_{k}M_{k}}{{H_{k}M_{k}}}$ end End of iterations.

where H_(k) represents a channel matrix of a user receiver k, H_(eff)represents an effective channel matrix of a user receiver that thereceiver actually experiences, H^(†) _(eff) represents apseudo-inversion matrix of the effective channel matrix H_(eff), M_(k)is a TX filter matrix for the user receiver k, W_(k) is an RX filtermatrix for the user receiver k, T denotes transposition, and * denotes acomplex conjugate transposition, N_(i) is a iteration number.

Because the channel matrixes change with time, the present inventionprovides an alternate algorithm. It can be used as a process forupdating the TX filters. In this algorithm, an initial value of a TXfilter is set to 1_(N)+i1_(N) where 1_(N) is a column vector with alength of N whose every entry is 1. Although the initial value is notoptimized, the performance is the same as in any initial value.Therefore, the initial value is easier to set up and the performance isthe same as in the previous computation algorithm. The alternatecomputation algorithm is comprised of the following steps:

Firstly, the TX filter is initialized to the value 1_(N)+i1_(N) for eachof the K users. For each user k, compute the matrix equal to the productof the transpose conjugate of the channel matrix of user k by thechannel matrix of user k.

Secondly, the following computation is repeated Ni times: Compute theeffective channel matrix using the current TX filters and the matricescomputed in the initialization step. Then compute the pseudo-inverse ofthe effective channel matrix. Then for each user k, update the currentTX filter as the k-th column of the inverted effective channel matrix.

Thirdly, normalize the TX filter for each user k. Then for each user k,compute the RX filter as the product of the channel matrix of user k byits TX filter, followed by normalization.

The alternate computation algorithm is summarized as follows:

Initialization for k = 1:K M_(k) = 1_(N) + i1_(N)$\overset{\_}{H_{k}} = {H_{k}^{*}H_{k}}$ end Repeat N_(i) times:$H_{eff} = \left\lbrack {\left( {M_{l}^{*}\overset{\_}{H_{l}}} \right)^{T}\;.\;.\;.\;\left( {M_{k}^{*}\overset{\_}{H_{k}}} \right)^{T}\;.\;.\;.\;\left( {M_{K}^{*}\overset{\_}{H_{K}}} \right)^{T}} \right\rbrack^{T}$M = H_(eff)^(†)(H_(eff)H_(eff)^(†))⁻¹ End of iterations. Normalize RXfilters for k = 1:K$\left. M_{k}\leftarrow\frac{M_{k}}{M_{k}} \right.$$W_{k} = \frac{H_{k}M_{k}}{{H_{k}M_{k}}}$ End

where H_(k) represents a channel matrix, H _(k) represents a modifiedmatched channel matrix, H_(eff) represents an effective channel matrixthat a user receiver actually experiences, M_(k) is a TX filter matrixfor a user receiver k, W_(k) is an RX filter matrix for the userreceiver k, T denotes transposition, and * denotes a complex conjugatetransposition, N_(i) is a iteration number.

The computation for the initialization is simple. In addition, if achannel change is sufficiently slow, the time-dependent performance canbe maintained by only a few number of iterations. The above algorithm isadvantageous in that not only the TX filter computation but also the RXfilter computation do not require the SVD. Also, the filter computationcomplexity of the above algorithm is even less than that of asingle-user closed-loop MIMO transmission scheme. Moreover, for allusers, the RX structure of the above algorithm is simpler than that of asingle-user open-loop layer transmission scheme. In addition, thetransmission efficiency of the above algorithm is much higher than thatof the single-user case.

Training Phase

The TX filter computed by the transmitter is transmitted to a receiverin a one-way channel sounding scheme. Instead of receiving an RX filtercomputed by the transmitter, the receiver receives the computed TXfilter over a channel and uses the product of the received TX filter anda channel matrix as an RX filter. Therefore, the receiver does notrequire channel estimation for computing an RX filter. A one-way channelsounding scheme is used to transmit the TX filter to the receiver. Inthe one-way channel sounding scheme, a DL symbol (e.g., a pilot signal)is used to transmit to the receiver the product of the TX filter and asequence signal (e.g., [1111]) that is agreed between the transmitterand the receiver. In another representation, the transmitter transmitsthe known-sequence (e.g., a pilot signal) through the TX filter of thetransmitter, and the receiver receives the known-sequence which waspassed thorough the TX filter and a downlink channel (the product of theTX filter and the downlink channel matrix).

As described above, a TX filter instead of an RX filter is transmittedto the user receivers. The receiver receives the TX filter over achannel and uses the product of the received TX filter and a channelmatrix to decode received data.

In the present invention, a normalization process is performed using thefollowing relationship between an RX filter and an optimal TX filter fora user receiver k.W_(k)˜a_(k)H_(k)M_(k)where a_(k) is a normalization parameter and is obtained innormalization process.

A pilot signal for the user receiver k is transmitted on a subcarrier k.At this point, a channel matrix is constant over several subcarriers.This corresponds to a case where a frequency band transmitted by asubcarrier is smaller than the coherence bandwidth of a channel. A pilotsignal transmitted by a base station (BS) can be detected by beamforminga sequence (signal) that is agreed between the BS and the user receiverk. For example, the agreed sequence (signal) may be a sequence of 1'sthat is transmitted with a power of P_(t). Another sequence may also bepossible. A TX beamforming vector is a TX filter for the user receiverk.

That is, the pilot signal transmitted to the user receiver k on thesubcarrier k is:x _(k) =M _(k)×1×√{square root over (P _(t))}=√{square root over (P_(t))}M _(k)

A signal received at the user receiver k on the subcarrier is expressedas:y _(k)=√{square root over (P _(t))}H _(k) M _(k) +w _(k)

where w_(k) is an Additive White Gaussian Noise (AWGN) vector ofN_(k)×1.

The above Equation can be rewritten as:y _(k)=√{square root over (P _(t))}H _(k) V _(k,N) +W _(k)

Then, a received signal vector is:y _(k) =∥H _(k) V _(k,N)∥√{square root over (P _(t))}W _(k) +W _(k)

The received signal is normnalized and used for estimation of an RXfilter.

${\hat{W}}_{k} = \frac{y_{k}}{y_{k}}$

This can be rewritten as:

${\hat{W}}_{k} = {\frac{{{{H_{k}V_{k,N}}}\sqrt{P_{t}}W_{k}} + w_{k}}{{{{{H_{k}V_{k,N}}}\sqrt{P_{t}}W_{k}} + w_{k}}} = \frac{W_{k} + \frac{w_{k}}{\sqrt{P_{t}{{H_{k}V_{k,N}}}}}}{{W_{k} + \frac{w_{k}}{\sqrt{P_{t}{{H_{k}V_{k,N}}}}}}}}$where H_(k) is a channel between the transmitter and the user receiverk, M_(k) is a TX matrix for the user receiver k, W_(k) is an RX matrixfor the user receiver k, P_(t) is TX power, and V_(k,n) is a singularvector at a null space of H_(stacked,k).

When there is no noise or the power of a pilot signal increases to ahigh level, Ŵ→W_(k).

Because of the nature of the pilot training process, user receivers donot need the estimation of their channel matrixes including NN_(k)complex coefficients. It is instead necessary to estimate each of N_(k)(the number of RX antennas) complex coefficients. These characteristicsreduce the RX complexity and increase the system bandwidth utilitybecause the channel estimation needs a small number of pilot signals.

These pilot signals may be transmitted to user receivers as a portion ofa data packet or as a preamble of a frame. The pilot signals may betransmitted over a dedicated channel or over a common channel.

This present invention can be easily applied to an Orthogonal FrequencyDivision Multiple Access (OFDMA) Time Division Duplexing (TDD) basedmulti-user MIMO channel system.

Referring to FIG. 4A, a transmitter performs a scheduling operation fordetermining to which user receiver data will be transmitted and anAdaptive Modulation and Coding (AMC) operation, in step 410. In step420, the transmitter computes a TX filter using the improved schemeaccording to the present invention. In step 430, the transmittertransmits the computed TX filter to a receiver in the one-way channelsounding scheme according to the present invention (SM TX filters 210,220 and 230 of the transmitter compute the TX filter). Although notillustrated, receivers 240, 250, 260 and 270 use the product of thereceived TX filter and a channel matrix as an RX filter. The receivernormalizes the RX filter according to Equation (1) and uses thenormalized RX filter. This normalization is performed in SM RX filters240, 250 and 260 of the receivers.

$\begin{matrix}{W_{k} = \frac{H_{k}M_{k}}{{H_{k}M_{k}}}} & (1)\end{matrix}$

Thereafter, a DL data transmission from the transmitter to the receiveris performed in step 440. The receiver decodes the received data usingthe above RX filter. After a while, the transmitter updates the TXfilter and transmits the updated TX filter to the receiver in theone-way channel sounding scheme, in step 450. The receiver normalizesthe product of the channel matrix and the TX filter received from thetransmitter and uses the normalization value as an RX filter.Thereafter, a DL data transmission from the transmitter to the receiveris performed in step 460. The receiver decodes the received data usingthe normalized RX filter.

Referring to FIG. 4B, a transmitter performs a scheduling operation fordetermining a user receiver to which data will be transmitted and anecessary AMC operation, in step 470. In step 471, the transmittercomputes a TX filter using the improved scheme according to the presentinvention. In step 480, the transmitter transmits the computed TX filterto a receiver in the one-way channel sounding scheme. This TX filtertransmission is performed simultaneously with a DL data transmission instep 490. Receivers 240, 250, 260 and 270 use the product of thereceived TX filter and a channel matrix as an RX filter. The receivernormalizes the RX filter and uses the normalized RX filter. Thisnormalization is performed in SM RX filters 240, 250 and 260 of thereceivers. After a while, the transmitter updates the TX filter andtransmits the updated TX filter to the receiver in the one-way channelsounding scheme, in step 481. The transmission of the updated TX filteris performed simultaneously with a next DL data transmission in step491.

The following parameters are considered for an actual design in theOFDMA system. It is necessary to find out the number of pilot signalsthat can be transmitted to each user receiver and thus to find out howmuch reliability is available in the estimation of optimal RX vectors. Avector quantization scheme is used to detect the number of resourcesnecessary for control signals. N user receivers are simultaneouslyserved with respective subcarriers over sub-bands for spanning thecoherence bandwidth of a channel.

-   -   The number of subcarriers: 1024    -   The coherence bandwidth: 256 subcarriers    -   The number of user receivers per coherence bandwidth=the        number (N) of TX antennas=4    -   The coding rate of control bit: R=1/20 to 1/8    -   The number of control bits for vector quantization based design:        B=6    -   The constellation used in control bits: QPSK

In order to transmit B bits to the user receivers per coherencebandwidth in a system where the control bit coding rate is R and thenumber of user receivers is N, it is necessary to transmit BN/R, i.e.,BN/(2R) QPSK symbols. If BN/(2R)<256, an operation is possible over oneOFDM symbol using BN/(2R) subcarriers per OFDM symbol. In the case ofthe same number of user receivers and the same coherence bandwidth, onlyone subcarrier is necessary that transmits a pilot signal having thesame average power as a data signal for the user receivers per coherencebandwidth. Therefore, N subcarriers are necessary that operate over oneOFDM symbol.

If an identical number of subcarriers are determined to be used as inthe previous case, a pilot signal can be transmitted (B/(2R)=BN/(2R)/N)times. In this case, a pilot TX power is raised by B/(2R) times. When areceiver performs a Linear Minimum Mean Square Error (LMMSE) operationon a pilot signal, the average SNR of the pilot signal can be increasedby 10 log₁₀(B/2R) dB over the average SNR of data signals. In addition,because the pilot can be increased up to 2.5 dB, the total increase canbe up to about (2.5+10 log₁₀(B/2R)) dB. For the foregoing parameters, anefficient pilot voltage increases by 16.3 dB to 20.3 dB for 1/8 to 1/20coding rates.

Data Transmission Phase

A BS transmitter transmits the product of a TX filter and a vector ofmodulation symbols s (where S_(k) is a symbol transmitted to a userreceiver k). Theses symbols are transmitted over a channel under thetotal TX power P₀.

The total TX power is equally allocated to N layers (from N TX antennasto the respective user receivers).

A transmitted vector is represented by:

$x = {\sqrt{\frac{P_{0}}{N}}{\sum\limits_{n = 1}^{N}{M_{n}s_{n}}}}$

A signal received by the user receiver k is represented by:

$y_{k} = {{\sqrt{\frac{P_{0}}{N}}H_{k}{\sum\limits_{n = 1}^{N}{M_{n}s_{n}}}} + w_{k}}$

The user receiver k multiplies the received signal by its RX filter toobtain

${{\hat{s}}_{k} = {{\hat{W}}_{k}^{*}y_{k}}},{{\hat{s}}_{k} = {{\hat{W}}_{k}^{*}\left( {{\sqrt{\frac{P_{0}}{N}}H_{k}{\sum\limits_{n = 1}^{N}{M_{n}s_{n}}}} + w_{n}} \right)}},{{\hat{s}}_{k} = {{\sqrt{\frac{P_{0}}{N}}{\sum\limits_{n = 1}^{N}{{\hat{W}}_{k}^{*}H_{k}M_{n}s_{n}}}} + {W_{k}^{*}{w_{n}.}}}}$

Theoretically, if

${{\hat{W}}_{k} = W_{k}},{{{\hat{W}}_{k}H_{k}M_{n}} = \left\{ {\begin{matrix}{0,{k \neq n}} \\{1,{k = n}}\end{matrix}.} \right.}$

Therefore,

${\hat{s}}_{k} = {{\sqrt{\frac{P_{0}}{N}}s_{k}} + w_{k}}$in theory.

where H_(k) is a channel between the transmitter and the user receiverk, M_(n) is a TX matrix for a user receiver n, W_(k) is an RX matrix forthe user receiver k, P₀ is the total TX power, and w_(k) and w_(n) areAWGNs.

On the assumption that user receivers can perfectly estimate RX filters,the BS can adapt the power allocated to each layer by using aconventional water filling scheme instead of a uniform power allocationscheme, thereby increasing the transmission efficiency.

Referring to FIG. 5A, a transmitter performs a scheduling operation fordetermining to which user receiver it will transmit data, in step 510.An Adaptive Modulation and Coding (AMC) operation may also be performedin step 510. In step 520, the transmitter computes a TX filter using theimproved scheme according to the present invention. In step 530, thetransmitter transmits the computed TX filter to a receiver using theone-way channel sounding scheme according to the present invention. Instep 540, the transmitter transmits data to the receiver.

The receiver multiplies the received TX filter by a channel matrix toobtain an RX filter, and normalizes the RX filter using Equation (1):

$\begin{matrix}{W_{k} = \frac{H_{k}M_{k}}{{H_{k}M_{k}}}} & (1)\end{matrix}$

In step 550, the receiver decodes the received data using the normalizedRX filter.

Referring to FIG. 5B, a transmitter performs a scheduling operation fordetermining to which user receiver it will transmit data, in step 560.An AMC operation may also be performed in step 560. In step 570, thetransmitter computes a TX filter using the improved scheme according tothe present invention. In step 580, the transmitter transmits thecomputed TX filter and data to a receiver using the one-way channelsounding scheme according to the present invention.

In step 590, the receiver multiplies the received TX filter by a channelmatrix to obtain an RX filter, normalizes the RX filter, and decodes thereceived data using the normalized RX filter.

Simulation Results

The performance of the present invention will now be examined in termsof the spectral efficiency using the Monte-Carlo simulation model.

FIG. 6 is a graph illustrating spectral efficiencies depending on thenumber of TX antennas in a multi-user multi-antenna communication systemusing the algorithm according to the present invention. The conditionsin FIG. 6 are identical to those in FIG. 3. The algorithm according tothe present invention can be implemented in an actual system. A dirtypaper coding algorithm is the theoretically best case, and a coordinatedbeamforming algorithm is an unrealistic case. As can be seen from FIG.6, the algorithm according to the present invention is very similar inperformance to the dirty paper coding algorithm and the coordinatedbeamforming algorithm.

FIG. 7 is a graph illustrating spectral efficiencies depending on dataSNRs in a multi-user multi-antenna communication system using analgorithm according to the present invention. FIG. 7 illustrates a casewhere a transmitter has four TX antennas and four receivers each havefour RX antennas. As can also be seen from FIG. 7, the algorithmaccording to the present invention is very similar in performance to thedirty paper coding algorithm and the coordinated beamforming algorithm.

As described above, the present invention provides a data transmittingapparatus and method for computing a TX filter of a transmitter in theimproved scheme and transmitting the computed TX filter to a receiver inthe one-way channel sounding scheme and a data receiving apparatus andmethod for receiving the TX filter over a channel and using the productof the received TX filter and a channel matrix as an RX filter.Accordingly, it is possible to reduce the system complexity. Inaddition, the receiver can be reduced in complexity because it does notrequire the channel estimation.

While the invention has been shown and described with reference tocertain preferred embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asfurther defined by the appended claims.

1. A transmitter for a multi-user multi-antenna communication system,the transmitter comprising: a Spatial Multiplexing (SM) transmit (TX)filter unit for computing TX filters for a plurality of user receivers,wherein each TX filter is computed in accordance with a pseudo-inversionof a respective effective channel matrix using a corresponding receive(RX) filter initially set to a random unitary vector; and a plurality ofantennas for transmitting the computed TX filters to the correspondinguser receivers.
 2. The transmitter of claim 1, wherein an N number ofthe TX filters are computed for an N number of the user receivers. 3.The transmitter of claim 1, wherein the SM TX filter unit furthercomputes RX.
 4. The transmitter of claim 1, wherein the transmitterdetermines a data transmission schedule for the user receivers andprovides the determined data transmission schedule to the SM TX filterunit.
 5. The transmitter of claim 1, wherein the channel matrixes areestimated using uplink sounding pilots from the user receivers.
 6. Thetransmitter of claim 1, wherein the channel matrixes are reported by theuser receivers.
 7. The transmitter of claim 1, wherein the TX filtersare computed using the following algorithm: Initialization for k = 1:KW_(k) = random unitary vector of size N_(k) × 1 end Repeat N_(i) times:Define  .H_(eff) = [(W_(l)^(*)H_(l))^(T) . . . (W_(k)^(*)H_(k))^(T) . . . (W_(K)^(*)H_(K))^(T)]^(T)Compute  .H_(eff)^(†) = H_(eff)^(†)(H_(eff)H_(eff)^(†))⁻¹ for k = 1:K$M_{k} = \frac{H_{{eff},\; k}^{\dagger}}{H_{{eff},\; k}^{\dagger}}$$W_{k} = \frac{H_{k}M_{k}}{{H_{k}M_{k}}}$ end End of iterations

where H_(eff) represents an effective channel matrix of a user receiverthat the receiver experiences, H^(†) _(eff) represents apseudo-inversion matrix of the effective channel matrix H_(eff), M_(k)is a TX filter matrix for a user receiver k, W_(k) is an RX filtermatrix for the user receiver k, T denotes transposition, and * denotes acomplex conjugate transposition.
 8. A transmitter for a multi-usermulti-antenna communication system, comprising: a Spatial Multiplexing(SM) transmit (TX) filter unit for computing a TX filter for each userreceiver, wherein each TX filter is computed in accordance with apseudo-inversion of a respective effective channel matrix using acorresponding receive (RX) filter initially set to a random unitaryvector; and a plurality of antennas for transmitting a training sequenceto each user receiver which are applied by the computed TX filter. 9.The transmitter of claim 8, wherein an N number of the TX filters arecomputed for an N number of the user receivers.
 10. The transmitter ofclaim 8, wherein the transmitter computes RX filters as well as the TXfilters.
 11. The transmitter of claim 8, wherein the transmitterdetermines a data transmission schedule for the user receivers, andprovides the determined data transmission schedule to the SM TX filterunit.
 12. The transmitter of claim 8, wherein the TX filters arecomputed using the following algorithm: Initialization for k = 1:K W_(k)= random unitary vector of size N_(k) × 1 end Repeat N_(i) times:Define  .H_(eff) = [(W_(l)^(*)H_(l))^(T) . . . (W_(k)^(*)H_(k))^(T) . . . (W_(K)^(*)H_(K))^(T)]^(T)Compute  .H_(eff)^(†) = H_(eff)^(†)(H_(eff)H_(eff)^(†))⁻¹ for k = 1:K$M_{k} = \frac{H_{{eff},k}^{\dagger}}{H_{{eff},k}^{\dagger}}$$W_{k} = \frac{H_{k}M_{k}}{{H_{k}M_{k}}}$ end End of iterations

where H_(eff) represents an effective channel matrix of a user receiverthat the receiver experiences, H^(†) _(eff) represents apseudo-inversion matrix of the effective channel matrix H_(eff), M_(k)is a TX filter matrix for a user receiver k, W_(k) is an RX filtermatrix for the user receiver k, T denotes transposition, and * denotes acomplex conjugate transposition.
 13. A method for transmitting data froma transmitter in a multi-user multi-antenna communication system, themethod comprising the steps of: computing transmit (TX) filters for aplurality of user receivers, wherein each TX filter is computed inaccordance with a pseudo-inversion of a respective effective channelmatrix using a corresponding receive (RX) filter initially set to arandom unitary vector; transmitting the computed TX filters through aplurality of antennas to the corresponding user receivers in a one-waychannel sounding scheme; and transmitting data to the user receivers.14. The method of claim 13, wherein an N number of the TX filters arecomputed for an N number of the user receivers.
 15. The method of claim13, wherein RX filters are computed simultaneously with the computationof the TX filters.
 16. The method of claim 13, further comprisingdetermining a data transmission schedule for the user receivers andproviding the determined data transmission schedule to a SpatialMultiplexing (SM) TX filter unit of the transmitter.
 17. The method ofclaim 13, wherein channel matrixes are estimated using uplink soundingpilots from the user receivers.
 18. The method of claim 13, whereinchannel matrixes are reported by the user receivers.
 19. The method ofclaim 13, wherein the TX filters are computed using the followingalgorithm: Initialization for k = 1:K W_(k) = random unitary vector ofsize N_(k) × 1 end Repeat N_(i) times:Define  .H_(eff) = [(W_(l)^(*)H_(l))^(T) . . . (W_(k)^(*)H_(k))^(T) . . . (W_(K)^(*)H_(K))^(T)]^(T)Compute  .H_(eff)^(†) = H_(eff)^(†)(H_(eff)H_(eff)^(†))⁻¹ for k = 1:K$M_{k} = \frac{H_{{eff},\; k}^{\dagger}}{H_{{eff},\; k}^{\dagger}}$$W_{k} = \frac{H_{k}M_{k}}{{H_{k}M_{k}}}$ end End of iterations

where H_(eff) represents an effective channel matrix of a user receiverthat the receiver experiences, H^(†) _(eff) represents apseudo-inversion matrix of the effective channel matrix H_(eff), M_(k)is a TX filter matrix for a user receiver k, W_(k) is an RX filtermatrix for the user receiver k, T denotes transposition, and * denotes acomplex conjugate transposition.
 20. A method for transmitting data froma transmitter in a multi-user multi-antenna communication system, themethod comprising the steps of: computing transmit (TX) filters for aplurality of user receivers, wherein each TX filter is computed inaccordance with a pseudo-inversion of a respective effective channelmatrix using a corresponding receive RX filter initially set to a randomunitary vector; and transmitting a training sequence through a pluralityof antennas which are applied by the computed TX filters to thecorresponding user receivers.
 21. The method of claim 20, wherein an Nnumber of the TX filters are computed for an N number of the userreceivers.
 22. The method of claim 20, wherein RX filters are computedwith the computation of the TX filters.
 23. The method of claim 20,further comprising determining a data transmission schedule for the userreceivers and providing the determined data transmission schedule to aSpatial Multiplexing (SM) TX filter unit of the transmitter.
 24. Themethod of claim 20, wherein the TX filters are computed using thefollowing algorithm: Initialization for k = 1:K W_(k) = random unitaryvector of size N_(k) × 1 end Repeat N_(i) times:Define  .H_(eff) = [(W_(l)^(*)H_(l))^(T) . . . (W_(k)^(*)H_(k))^(T) . . . (W_(K)^(*)H_(K))^(T)]^(T)Compute  .H_(eff)^(†) = H_(eff)^(†)(H_(eff)H_(eff)^(†))⁻¹ for k = 1:K$M_{k} = \frac{H_{{eff},\; k}^{\dagger}}{H_{{eff},\; k}^{\dagger}}$$W_{k} = \frac{H_{k}M_{k}}{{H_{k}M_{k}}}$ end End of iterations

where H_(eff) represents an effective channel matrix of a user receiverthat the receiver experiences, H^(†) _(eff) represents apseudo-inversion matrix of the effective channel matrix H_(eff), M_(k)is a TX filter matrix for a user receiver k, W_(k) is an RX filtermatrix for the user receiver k, T denotes transposition, and * denotes acomplex conjugate transposition.